Wireless communication systems are well known in the art. In previous wireless communication systems, radio interface is usually a bottleneck due to its limited transmitting capacity, and researches mainly focus on the impact from radio layer. But this is not necessarily true in advanced wireless communication systems, e.g. the third generation (3G) wireless communication systems, such as WCDMA, TD-SCDMA, CDMA2000 and their evolutions, because these wireless communication systems could provide large bandwidth even in radio interface layer. In the Third Generation Partnership Project (3GPP) High Speed Packet Access (HSPA) scope, a 2 by 2 Multiple Input Multiple Output (MIMO) is introduced in Rel-7, which means that there are two transmitting antennas in Node B side and two receiving antennas in User Equipment (UE) side. With 2 by 2 MIMO, up to two streams can be transmitted in parallel, and then the radio interface is able to provide even larger bandwidth and no longer a bottleneck in wireless communication.
With the rapid development of air interface technology, instead of radio interface, bandwidth of upper layers and/or application server gradually becomes the bottleneck in wireless communication. The bottleneck caused by upper layers may be, for example:                The bandwidth limit of backbone trunk;        The bandwidth limit of Iub tunnel;        The window stalling of Transport Control Protocol (TCP)/Radio Link Control (RLC); and        The bandwidth limit of application server.        
In upper layers, there are already scheduling and congestion mechanisms about the bandwidth. For instance, in the backbone network, the field of Differentiated Services Code Point (DSCP) in Internet Protocol (IP) header is used to indicate the service priority to use the shared bandwidth, and in Iub interface, the flow control functionality is used to schedule the bandwidth and handle the congestion. This means the performance of upper layers plays a more and more important role in wireless communication systems and the interface between MAC and physical layer should consider the performance of upper layers.
Medium Access Control (MAC) layer is responsible for mapping logical channels to transport channels and selecting appropriate Transport Format for each transport channel. Data from upper layers is preferably buffered in a transmit buffer in MAC layer before TFs are selected for transport channels. A bottleneck in upper layers may render upper layers unable to supply sufficient data to the transmit buffer and will lead to a buffer limitation case.
Buffer limitation as used herein after refers to a situation in multiple-stream transmission where data quantity buffered in the transmit buffer is lower than the transport capacity of the multiple-streams.
Given a 2×2 MIMO transmission scheme in compliance with 3GPP specification as an example, when in dual-stream transmission case, TFs may be selected for streams, and same High Speed-Physical Downlink Shared Channel (HS-PDSCH) codes may be forced to use for both streams. During TF selection, a TF may firstly be selected for the first stream by taking total data bits buffered in a transmit buffer into account, and a maximum capable TF, which is the TF with the maximum transport block size with which the first stream can transmit, may be selected for the first stream. When selecting TF for the second stream, the buffered data bits to be considered may be rest data bits, i.e. the remaining data bits when subtracting the data bits to be transmitted by the first stream from the total buffered data bits. This scheme works well if there is no bottleneck in the upper layers, i.e. there are sufficient data bits buffered in the transmit buffer and the selected TFs best match the transmitting capacities of the two streams.
However, in case of buffer limitation, that is, if the bandwidth of upper layer becomes a bottleneck, it is possible that the data bits to be transmitted by the second stream will be much lower than its transmitting capacity. This becomes a problem.
When the TF selection is performed sequentially for two streams, the first stream may be allocated too many data bits to transmit as its TF is selected by taking total buffered data bits into account, and few data bits are left for transmitting by the second stream. This will lead to performance loss at least in following aspects:
1. Waste of HS-PDSCH Codes
A TF with large TB size is selected for the first stream. Since the number of HS-PDSCH codes to be used is determined by the TF, a large number of HS-PDSCH codes is needed to transmit such a large transport block, and the second stream will have to use same HS-PDSCH codes with small TB size, which means that there will be less HS-PDSCH codes left for other HS-DPA (High Speed-Downlink Packet Access) UEs to be scheduled in same cell. This causes a waste of HS-PDSCH code resource.
2. Larger Power/CQI Back-Off
The first stream uses much higher transmit power than the second stream statistically, which makes the interference from the first stream to the second stream much higher than the interference from the second stream to the first stream. In order to conquer such an inter-stream interference imbalance to reach a predetermined Block Error Ratio (BLER) target, the CQI adjustment needs larger back-off, which means a larger power backoff and a lower power utilization efficiency.
3. Transmit (TX) Power Imbalance Between Antennas
When utilizing a common pre-coder, the power imbalance between streams can result in the power imbalance between two TX antennas when two of the four Dual-stream Transmit Antenna Array (D-TxAA) pre-coders are used.
Another minor performance loss is due to the protocol/padding overhead when dual stream transmission is used in case there are too little buffered data bits to be transmitted.